1. Field of the Invention
The present invention relates to a mobile terminal, a control device, a communication system, and a communication method.
2. Description of the Related Art
There has been demand for mobile terminals with long continuous communication periods and continuous stand-by periods. However, since a mobile terminal moves between cells covered by a plurality of base stations, the mobile terminal must find and connect to a base station that covers the cell in which the mobile terminal exists. The mobile terminal must receive a broadcast signal from a base station in order to find the base station. As a result, the mobile terminal exhausts its battery, and the continuous stand-by period becomes shorter. Consequently, a mobile terminal has been proposed, which reduces power consumption and lengthens the continuous stand-by period by decreasing the number of times of receiving broadcast signals according to the assumption that there is little need to find a base station, since the same base station continues to connect and serve stationary mobile terminal.
For example, a mobile terminal 30 as shown in FIG. 1, which utilizes the fact that the speed of variation in phasing caused by movement of a mobile terminal varies depending on the mobile terminal movement speed, has been proposed. A broadcast signal receiver 31 receives broadcast signals from base stations. The mobile terminal 30 obtains base station information from the received broadcast signals. A reception period determination unit 32 measures a maximum Doppler frequency based on the reception signal received by the broadcast signal receiver 31. If the maximum Doppler frequency is high, the reception period determination unit 32 then determines that the movement speed is fast, and shortens the period for receiving the broadcast signals. Meanwhile, if the maximum Doppler frequency is low, the reception period determination unit 32 determines that the movement speed is slow, and lengthens the period for receiving the broadcast signals.
In addition, a mobile terminal 40 as shown in FIG. 2, which uses a vibration sensor so as to measure mobile terminal movement state, has been proposed (Japanese Patent Laid-Open No. 2000-101508). The mobile terminal 40 includes a radio unit 41, a voice coding unit 42, a transmitter/receiver 43, a control device 44, and a vibration sensor 45. The control device 44 switches between a normal power saving mode in which all broadcast channels for call arrival are received while in stand-by and a high-degree power saving mode in which broadcast channels for call arrival are received intermittently while in stand-by, based on information from the vibration sensor 45. In addition, a mobile terminal has been proposed, which determines whether or not the reception state of a control channel is good, and which controls the reception interval of the control channels for call arrival in a stand-by state, based on that determination result (Japanese Patent Laid-Open No. Hei 9-261153).
Nevertheless, the conventional mobile terminal controls the control signal reception period with consideration for only the movement state or with consideration for only the reception state. In addition, the control signal reception period is determined without consideration for the communication state of the mobile terminal. Therefore, the control signal reception period may not be appropriately controlled. As a result, the mobile terminal may not sufficiently reduce power consumption or appropriately receive control signals. Furthermore, the conventional mobile terminal controls the control signal reception period only while in stand-by, and does not reduce power consumption while in communication. Accordingly, the power consumption of the mobile terminal may not be sufficiently reduced.
In addition, since the maximum Doppler frequency may not be determined based on only the actual movement speed of the mobile terminal, with a method for controlling reception period based on the maximum Doppler frequency, the control signal reception period may not be appropriately controlled. This is because there are cases where phasing temporally varies according to the movement of surrounding objects and Doppler shift occurs even if the mobile terminal is stationary, therefore preventing correct measurement of the movement state of the mobile terminal. As a result, there are cases where the number of times of receiving broadcast signals is not sufficiently reduced due to the mobile terminal being determined as moving despite actually being stationary. In addition, the mobile terminal must activate the broadcast signal receiver 31 in order to measure the Doppler frequency. As a result, mobile terminals may not reduce power consumption sufficiently.
Furthermore, the control signal for call arrival must be received somewhat frequently in order to prevent lengthening of the time required for call arrival. Accordingly, in the case of controlling the reception interval of the control signal for call arrival, there are restrictions where reception must be performed frequently so as to prevent lengthening of the time required for call arrival. Therefore, there have been cases where power consumption could not be widely reduced.